Why transition elements show colour?
It is all due to electronic excitation energies that happen to fall in the visible spectrum. Usually, a transition metal ion will have ligands coordinated to it and this will disrupt the order of the d-orbitals. For instance, in an octahedral complex all the ligands are bonded directly to the d(x2-y2) and d(z2) orbitals and they are known as the eg set. Due to the increase in electron density in the eg set, they are raised in energy relative to the d(xy), d(xz), and d(yz) orbitals, also known as the t2g set. The energy difference between the two sets is referred to as delta octahedron and its energy falls somewhere in the visible spectrum. Thus, an electronic transition from the t2g set to the eg set will require absorption of light matching that energy and the transmitted light (what we see) will be the remainder of wavelengths that we see as color.
